What are the not so obvious features of Rigify?
Let me start by saying that I think that the finger curl trick might have mislead you a bit. As complicated and advanced a mechanism as Rigify is, it's nothing really 'unique' in terms of rigging in Blender and rigging in general and as Fweeb says above..

...for people who have been using rigs in Blender for a long time, scale controls on the
fingers are a long-standing convention that doesn't feel alien at all.

Apart from the finger curl when scaled, there isn't really anything much left to 'discover' per se, it is just a nice collection of common operations etc. and control interfaces to give animators more control and allow people to skip most of the arduous work when rigging as it can be expanded on if needed.
You might know most if not all of these already.. and I won't attempt to explain Rigify in full detail, that would be very difficult to do and is beyond the scope of this question. — However, as a future reference for beginners…

The top (n) or so things that are not so obvious at first glance about Rigify are:
1. This is not the rig you are looking for…

The very first thing is that Rigify is not really the default armature you get when you add it to a scene. To get the full rig, you go to the Armature: Object Data panel and under Rigify Buttons hit Generate. After that, you can just move the default meta-rig to another layer to keep it out of your way.



2. Now you see me, now you don't

Rigify is a complicated mechanism and often times might get in the way of what you are doing, to hide different parts of the rig as they are needed, you can open the Rig Layers group in the Properties panel and toggle the visibility of each layer of controls as needed.



3. FK or IK?

The rig comes ready to be used in any way you want to animate. It has sliders to change between Forward Kinematics and Inverse Kinematics or give you a blend of both and more depending on how you adjust the slider(s) under Properties and or Rig Main Properties as they are presented or made avilable in the Properties panel.



4. Those aren't really bones..

The custom handles you see aren't really bones per se, they are bones with custom shapes and are actually objects that you can modify and by default they are placed on the last layer (layer 20).

These can also be changed using the Custom Shape input box under Display in the Bones panel but it is worth noting that modifying these can lead to problems with your rig later on so be careful.



5. Bits and pieces

You can encode the meta-rig to python code so that you can generate select parts of the rig as needed. Look in the toolshelf under Rigify Dev Tools and experiment with this.

6. Wait.. who's driving?

This is out of the scope of the question but might be useful if you're planning to build your own rig. Rigify makes extensive use of drivers to automate and make certain operations more 'smooth' etc.

At any time, you can poke around and select a bone and if any of the transform panels turn purple (the default color if you haven't changed it) this means that a driver is acting on it. To dig into this, just open the graph editor and open the Drivers view.

It is also worth noting that since Rigify is glued together by scripts and not manually put together for the most part, a lot of these features will be hard to trace and pull apart etc. but can serve as good reference nonetheless.



Most of these explanations are a bit vague and short, it is a bit more indepth than this but this is enough to get someone new to the rig at least a bit more familiar with it… and so as a last note, I would suggest reading up on Bone Constraints, Drivers and Properties.
Also related.. Armature FK/IK toggles